1H, 13C and 15N NMR spectroscopy and tautomerism of nitrobenzotriazoles

Benzotriazole nitro derivatives were prepared by nitration of the corresponding benzotriazoles and by methylation or cyclization of appropriate nitro‐1,2‐phenylenediamines. Structures and tautomerism of the nitrobenzotriazoles were studied by multinuclear ¹H, ¹³C, ¹⁵N, and 2D NMR spectroscopy and quantum chemistry. Copyright © 2008 John Wiley & Sons, Ltd.     

FT-IR, UV-vis, 1H and 13C NMR spectra and the equilibrium structure of organic dye molecule disperse red 1 acrylate: a combined experimental and theoretical analysis

This study reports the characterization of disperse red 1 acrylate compound by spectral techniques and quantum chemical calculations. The spectroscopic properties were analyzed by FT-IR, UV-vis, (1)H NMR and (13)C NMR techniques. FT-IR spectrum in solid state was recorded in the region 4000-400 cm(-1). The UV-vis absorption spectrum of the compound that dissolved in methanol was recorded in the range of 200-800 nm. The (1)H and (13)C NMR spectra were recorded in CDCl(3) solution. The structural and spectroscopic data of the molecule in the ground state were calculated using density functional theory (DFT) employing B3LYP exchange correlation and the 6-311++G(d,p) basis set. The vibrational wavenumbers were calculated and scaled values were compared with experimental FT-IR spectrum. A satisfactory consistency between the experimental and theoretical spectra was obtained and it shows that the hybrid DFT method is very useful in predicting accurate vibrational structure, especially for high-frequency region. The complete assignments were performed on the basis of the experimental results and total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. Isotropic chemical shifts were calculated using the gauge-invariant atomic orbital (GIAO) method. A study on the electronic properties were performed by timedependent DFT (TD-DFT) and CIS(D) approach. To investigate non linear optical properties, the electric dipole moment μ, polarizability α, anisotropy of polarizability Δα and molecular first hyperpolarizability β were computed. The linear polarizabilities and first hyperpolarizabilities of the studied molecule indicate that the compound can be a good candidate of nonlinear optical materials.     

The tautomerism of Omeprazole in solution: a 1H and 13C NMR study

The tautomerism of 5(6)-methoxy-2-([(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl] sulfinyl)-1H-benzimidazole (omeprazole) was determined in solution, K(T) = 0.59 in THF at 195 K, in favor of the 6-methoxy tautomer. The assignment of the signals was made by comparison with its two N-methyl derivatives in acetone-d6 and through theoretical calculations of the absolute shieldings (GIAO/DFT/6-311++G**).     

Tumbling motions of NH2(CH3)2 ions in [NH2(CH3)2]2ZnCl4 studied using 1H MAS NMR and 13C CP/MAS NMR

The structure and the phase transition temperatures of [NH₂(CH₃)₂]₂ZnCl₄ were determined using X-ray diffraction and DSC, respectively. The temperature dependence of chemical shifts and the spin–lattice relaxation time T_1ρ in the rotating frame were measured for the ¹H and ¹³C nuclei in [NH₂(CH₃)₂]₂ZnCl₄. From these results, it was observed that the structural change by chemical shifts does not occur with temperature. However, T_1ρ for ¹H and ¹³C in [NH₂(CH₃)₂]₂ZnCl₄ showed a minimum, and it is apparent that both T_1ρ values are governed by the same tumbling motions. The activation energies of tumbling motions for ¹H and ¹³C are nearly the same owing to the connection between CH₃ and NH₂ ions in the [NH₂(CH₃)₂]⁺ group.     

1H and 13C NMR study of azido-tetrazole tautomerism of 2-azido-4-methylpyrimidine

Using ¹H and ¹³C NMR and IR spectroscopic methods, it was found that 2-azido-4-methylpyrimidine exists in solutions in tautomeric equilibrium with two tetrazole forms, the ratio between which is determined by the polarity of the solvent, while in a crystalline state, according to the ¹³C NMR CP MAS data it has the structure of the 7-methyltetrazolo[1,5-a]pyrimidine isomer.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1648–1654, December, 1990.     

1H and 13C NMR studies on 1,3,2-dioxarsolanes

The ¹H and ¹³C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the ¹H and proton-coupled ¹³C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from ¹H or ¹³C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal ¹H? ¹H and ¹³C? ¹H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.     

Unimolecular reactions of the isolated immonium ions CH3CH = NH+C4H9, CH3CH2Ch = NH+C4H9 and (CH3)2C = NH+C4H9

The reactions of ten metastable immonium ions of general structure R¹R²C?NH⁺C₄H₉ (R¹ = H, R² = CH₃, C₂H₅; R¹ = R² = CH₃) are reported and discussed. Elimination of C₄H₈ is usually the dominant fragmentation pathway. This process gives rise to a Gaussian metastable peak; it is interpreted in terms of a mechanism involving ion-neutral complexes containing incipient butyl) cations. Metastable immonium ions ontaining an isobutyl group are unique in undergoing a minor amount of imine (R¹R²C?NH) loss. This decomposition route, which also produces a Gaussian metastable peak, decreases in importance as the basicity of the imine increases. The correlation between imine loss and the presence of an isobutyl group is rationalized by the rearrangement of the appropriate ion-neutral complexes in which there are isobutyl cations to the isomeric complexes containing the thermodynamically more stable tert-butyl cations. A sizeable amount of a third reaction, expulsion of C₃H₆, is observed for metastable n-C₄H₉ ⁺NH?CR¹R² ions; in contrast to C₄H₈ and R¹R²C?NH loss, C₃H₆ elimination occurs with a large kinetic energy release (40–48 kJ mol^?1) and is evidenced by a dish-topped metastable peak. This process is explained using a two-step mechanism involving a 1,5-hydride shift, followed by cleavage of the resultant secondary open-chain cations, CH₃CH⁺ CH₂CH₂NHCHR¹R².     

Topotecan dynamics,tautomerism and reactivity—1H/13C NMR and ESI MS study

Topotecan (TPT) is in clinical use as an antitumor agent, hycamtin^?. Because of this, it requires both biologically and chemically useful information to be available. TPT acts by binding to the covalent complex formed by nicked DNA and topoisomerase I. This has a poisonous effect since inserted into the single‐strand nick and TPT inhibits its religation. We used NMR to trace TPT dynamics, tautomerism and solvolysis products in various solvents and conditions. Chemical stability was assessed in methanol and DMSO as compared to water, and the regioselectivity of the N‐ and O‐methylation was studied using various alkylating agents. The reaction products of quaternization of the nitrogen atom and methylation of the oxygen atom were characterized by means of ESI MS, ¹H/¹³C‐HMBC and ‐HSQCAD NMR. We have focused on the NMR characterization of TPT with an anticipation that its aggregation, tumbling properties and the intramolecular dipolar interactions will be a common feature for other compounds described in this article. These features can also be useful in tracing the interactions of this class of topoisomerase I (TopoI) poisons with DNA. Moreover, the results explained shed light on the recently disclosed problem of lack of stability of TPT in the heart tissue homogenate samples using the analytical assays developed for this class of compounds carried out in the presence of methanol. Copyright © 2010 John Wiley & Sons, Ltd.     

13C and 1H NMR spectral studies of N-alkylmethylquinolinium salts

¹³C and ¹H chemical shifts of fourteen N-alkylmethylquinolinium salts in DMSO-d₆ are reported, and compared with those of the eleven corresponding methylquinoline bases. The influence of ring substitution by methyl groups in the salts and substitution at the nitrogen atom and the effect of the anion are discussed.     

1H, 13C and 19F NMR studies on fluorinated ethers

The enflurane and ethoxyflurane ¹H, ¹³C and ¹⁹F NMR spectra are examined—including sign determination of FF and FH couplings—and considered in the light of previously reported results for methoxyflurane. Conformational differences between methoxyflurane and the former two molecules are indicated by through space FH coupling constants and by the nonequivalence of geminal fluorine nuclei. Populations of conformers about the CC bond are estimated.     

Computational 1H and 13C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹H and ¹³C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin–spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹H and ¹³C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.     

1H and 13C NMR studies of some germacrones and isogermacrones

The ¹H and ¹³C NMR spectra of the trans,trans-, cis,cis- and cis-C-3–C-4, trans-C-7–C-8-germacrones and of the cis-C-2–C-3, trans-C-7–C–8, trans-C-2–C-3, cis-C-7–C-8- and cis,cis-isogermacrones are analysed. The last two isogermacrones are new compounds. The C-2–C-3 double bond in the previously described isogermacrone is found to be of cis configuration, contrary to the hitherto accepted trans arrangement.     

1H, 13C NMR, X-ray and conformational studies of new 1-alkyl-3-benzoyl-pyrazole and 1-alkyl-3-benzoyl-pyrazoline derivatives

The (1)H and (13)C NMR resonances of 22 1-alkyl-pyrazole and 25 1-alkyl-pyrazoline derivatives were assigned completely using the concerted application of one- and two-dimensional experiments (DEPT, gs-HMQC and gs-HMBC). Nuclear Overhauser enhancement (NOE) effects, conformational analysis and X-ray crystallography confirm the preferred conformation of those compounds.     

1H and 13C NMR studies on 3-aryl-2-thioxo-4-oxazolidinones and 3-arylrhodanines

3-Aryl-2-thioxo-4-oxazolidinones and 3-arylrhodanines have been studied for magnetic non-equivalence of diastereotopically related proton and ¹³C nuclei in rotational isomers, and for steric interactions between the aryl and heterocyclic moieties of these compounds. For the majority of rotational isomers the barriers to internal rotation about the aryl C? N bond were >100 kJ mol^?1, due to the steric bulk of the thiocarbonyl group. Chemical isolation of several of the diastereomers was achieved. The enhanced steric effect and the difference in the electronic effect of the sulphur atom in relation to the oxygen atom appeared to have no influence on the small chemical shift differences of the rotational isomers, detected for some ¹H and some ¹³C nuclei.     

Structural Revision and Elucidation of the Biosynthesis of Hypodoratoxide by 13C,13C COSY NMR Spectroscopy

Feeding of (2,3,4,5,6‐¹³C₅)mevalonolactone to the fungus Hypomyces odoratus resulted in a completely labeled sesquiterpene ether. The connectivity of the carbon atoms was easily deduced from a ¹³C,¹³C COSY spectrum, revealing a structure that was different from the previously reported structure of hypodoratoxide, even though the reported ¹³C NMR data matched. A structural revision of hypodoratoxide is thus presented. Its absolute configuration was tentatively assigned from its co‐metabolite cis‐dihydroagarofuran. Its biosynthesis was investigated by feeding of (3‐¹³C)‐ and (4,6‐¹³C₂)mevalonolactone, which gave insights into the complex rearrangement of the carbon skeleton during terpene cyclization by analysis of the ¹³C,¹³C couplings.     

Experimental studies of the 13C NMR of iodoalkynes in Lewis-basic solvents

The (13)C NMR spectra of two different iodoalkynes, 1-iodo-1-hexyne (1) and diiodoethyne (2), exhibit a strong solvent dependence. Comparisons of the data with several common empirical models, including Gutmann's Donor numbers, Reichardt's E(N)(T), and Taft and Kamlet's beta and pi, demonstrate that this solvent effect arises from a specific acid-base interaction. Solvent basicity measures such as Donor numbers and beta values correlate well with the alpha-carbon chemical shift of 1, but polarity measures such as E(N)(T) and pi do not correlate. The similarity of the solvent effect for 1 and 2 suggests that carbon-carbon bond polarization may not play a role in the change in chemical shift, as previously hypothesized.     

13C and 1H NMR spectroscopic studies on the structure of N-methyl-3-pyridone and 3-hydroxypyridine

Carbon and proton NMR spectra of 3-hydroxypyridine and its O-methyl and N-methyl derivatives have been analysed, and are compared with the spectra of the corresponding 2- and 4-substituted pyridines. It is shown, that contrary to quantum chemical predictions and in agreement with chemical experience, 3-hydroxy-pyridine has a phenolic structure in solution. For the N-methyl derivative, however, carbon and proton NMR data clearly demonstrate that this compound should be formulated as N-methyl-3-pyridone. The general problem of the 3-pyridones is discussed on the basis of calculated π-electron densities, IR and NMR data.     

1H and 13C NMR studies of 7-azaindole and related compounds

The ¹H and ¹³C chemical shifts, proton-proton coupling constants, and one-bond carbon-hydrogen coupling constants have been obtained for 7-azaindole, 1-methyl-7-azaindole, their corresponding methyl iodide salts, and the related compound 7-methyl-7H-pyrrolo [2,3-b]pyridine. are different from those of either 7-azaindole or 1-methyl-7-azaindole.